1. Field of the Invention
The present invention relates generally to the optical transmission field. More particularly, but not by way of limitation, the present invention relates to a method and apparatus for compensating for first-order Polarization Mode Dispersion in an optical transmission system.
2. Description of the Prior Art
Polarization Mode Dispersion (PMD) is the residual dispersion that arises in optical fibers because the degeneracy of the polarization modes within the fundamental spatial mode is leveled by small asymmetric stresses along the fiber. This dispersion may be sufficiently high to induce eye-closure penalties in an optical transmission system incorporating an optical fiber. When a sharp spectrum optical signal, such as is employed, for example, in WDM (Wavelength Division Multiplexing) transmissions, travels along an optical fiber; the signal can be described as being composed of the fastest and the slowest polarization modes, also called the Principal States of Polarization (PSP). The differential group delay between these states is variable as a result of random variations in external conditions imposed on the optical fiber; for example, variations in temperature, humidity and the like. As a result, polarization fading of a signal transmitted through the optical transmission system may be observed.
One proposed solution to the problem of fading in an optical transmission system is to always launch the light in one of the PSPs (see T. Ono, S. Yamazaki, H. Shimizu and H. Emura, Polarization Control Method for Suppressing Polarisation Mode Dispersion in Optical Transmission Systems, Journal of Lightwave Technology, vol. 12, pages 891-898, 1994). However, measuring or monitoring the input PSP requires access to and communication between both ends of the optical fiber; and, in many applications, this can present a significant problem.
In general, existing methods for avoiding fading in an optical transmission system encounter two related problems, random fluctuations of the Principal States of Polarization and random fluctuations of the Differential Group Delay. One known method that partly solves the fading problem consists of introducing a compensator in the form of a single linear birefringence element into the transmission system, for example, a piece of High Birefringence optical fiber, that has a delay that equals the PMD of the fiber, which is the mean value of the Differential Group Delays (DGD) along the fiber (see T. Takahashi, T. Imai and M. Aiki, Automatic Compensation Technique for Timewise Fluctuating Polarization Mode Dispersion in In-line amplifier Systems, Electronics Letters, vol. 30, no. 4, page 348, February, 1994; and R. Noe, D. Sandel, M. Yoshida-Dierolf and Others, Polarization Mode Dispersion Compensation at 10, 20 and 40 Gb/s with various Optical Equalizers, Journal of Lightwave Technology, vol. 17, no. 9, page 1602, September, 1999). A polarization transformer matches the PSP of the fiber to those of the delay element in such a way as to subtract their delays one from the other.
A disadvantage of this method is that only the mean value of the DGD is compensated for. Moreover, when the DGD of the fiber becomes small, the compensator will actually degrade the performance of the optical transmission system.
The use of a variable delay element can overcome these disadvantages (see, for example, the R. Noe et al publication identified above, as well as B. W. Hakki, Polarization Mode Dispersion Compensation By Phase Diversity Detection, IEEE Photonics Technology Letters, vol. 9, no. 1, page 121, January, 1997; F. Heismann, D. A. Fishman and D. L. Wilson, Automatic Compensation of First-Order Polarization Mode Dispersion in a 10 Gb/s Transmission System, ECOC'98, Madrid, Spain, pages 529-530, September, 1998; and U.S. Pat. No. 5,930,414). A problem that still remains with this approach, however, is how to control the polarization transformer to automatically perform adjustment of the axes. One proposed solution involves splitting a part of the optical signal at the receiver and measuring its degree of polarization (see S. Lanne, J-P. Thiery, D. Pennickx, J-P. Hamaide, J-P. Soigne, B. Desthieux, J. Le Briand, L. Mace and P. Gavignet, Field Optical PMD Compensation at 10 Ghb/s Over Installed Fiber Totaling 35 ps of PMD, ECOC 2000, Munich, Germany, pages 207-208, September, 2000).
U.S. Pat. No. 5,930,414 referred to above proposes yet another solution to the problem. This patent describes a method and apparatus for automatic compensation of first-order PMD based on the introduction of a variable linear birefringence element acting as a variable time delay line at the end of an optical transmission system. A polarization transformer aligns the output principal states of the fiber to the axes of the variable delay line. Alignment is automatically accomplished based on measurement of the amplitude of a plurality of frequency components contained in the optical information signal propagating through the fiber. This procedure avoids ambiguity in the control system, which could track the polarization state to a situation corresponding to a local maximum of the RF power instead of the absolute maximum.
In general, problems arise in many existing compensation methods when the states of polarization of the optical transmission system change rapidly; and limits in the response time of the compensator restricts its ability to follow these rapid changes. Frequently, the response time can be too long for the optical transmission system to maintain its synchronization.
The principal factors that limit the response time in various proposed compensation apparatus include the response time of materials used in actuators of the apparatus, and the complexity of the algorithm used to control the operation of the apparatus. In this regard, known compensation apparatus usually measure a few parameters which are then used as feedback signals to the actuators. Normally, the feedback signals come from spectral filtering (power signal in 1/(2T) and 1/(4T), where T is the period of the clock frequency of the system), bit error rate or degree of polarization. For example, in U.S. Pat. No. 5,930,414, all actuators are modulated at the same dithering frequency and are distinguished from one another through the phase of the detected signal. The number of independent parameters is necessarily smaller than the number of actuators if more than two actuators are used. Synchronizing pairs of actuators overcomes this difficulty if there are three or more actuators.
There is, accordingly, a need for a method and apparatus for compensating for Polarization Mode Dispersion in an optical transmission system that provides for a reduction in response time of the actuators used in the apparatus and a reduction in the over-all complexity of the algorithm used to control the apparatus.